A weir on the Yass River, New South Wales, Australia directly upstream from a shared pedestrian-bicycle river crossing. There are many designs of weir, but commonly water flows freely over the top of the weir crest before cascading down to a lower level. There is no single definition as to what constitutes a weir and one English dictionary simply defines a weir as a small dam, likely originating weir design calculations pdf Middle English were, Old English wer, derivative of root of werian, meaning “to defend, dam”.
Weirs are commonly used to prevent flooding, measure water discharge and help render rivers more navigable by boat. In some locations the terms dam and weir are synonymous, but normally there is a clear distinction made between the structures. A dam is usually specifically designed to impound water behind a wall, whilst a weir is designed to alter the river flow characteristics. Accordingly, the crest of an overflow spillway on a large dam may therefore be referred to as a weir.
Weirs can vary in size both horizontally and vertically, with the smallest being only a few inches in height whilst the largest may be hundreds of metres long and many metres tall. Some common weir purposes are outlined below. Since the geometry of the top of the weir is known and all water flows over the weir, the depth of water behind the weir can be converted to a rate of flow. If these conditions are not met, it can make flow measurement complicated, inaccurate or even impossible.